In general, in order to achieve high strength and efficiency, light weight shell or cylindrical structures must be stiffened with longerons and frames. Current design practices utilize mechanical means, such as rivets and bolts, or adhesive bonding to tie the structural elements together. It has been determined that structural components made with the least number of small stiffener parts substantially reduces the cost of the structure. However, in metal structures, in order to reduce the number of stiffener parts employed, it is necessary to integrally machine the stiffeners into the structural components or panels from thick metal plates.
The use of structural composites such as graphite-epoxy composites in structural components recently has assumed considerable importance as weight and stiffness become the motivating criteria for such designs. A recently introduced stiffening configuration called "isogrid" has been shown to result in a highly reduced weight structure, while providing sufficient stiffening. Generally such isogrid configurations are comprised of a plurality of webs or grids, such stiffening members being arranged in a pattern with the respective members oriented at an angle, e.g. 60.degree., to each other. The advantage of employing such an isogrid shear web as a stiffening member for structural components is that such a stiffener system can be manufactured at low cost in one piece from either metal or composites.
It is an object of the present invention to fabricate composite structural members employing isogrid stiffening. A particular object is to fabricate such composite structures employing compression molding coupled with co-curing, that is, simultaneous curing of stiffener and structural member, to integrally bond an isogrid stiffener or shear web to a structural component, for example the inside of a shell or cylinder, preferably also formed of a composite, particularly a composite comprised of layers of certain pre-impregnated tape.